1. Field of the Invention
The present invention relates to improvements in apparatus for aspirating liquids, e.g., whole blood, from a sample container and for dispensing precise volumes of the aspirated liquid to multiple reaction chambers and/or baths for subsequent mixing in order to prepare the aspirated liquid for analysis or further processing, as appropriate. The invention is particularly useful in the fields of hematology, flow cytometry and blood chemistry in which it is often necessary to dispense, with high precision, relatively tiny volumes (e.g., 1-30 microliters) of whole blood and/or a prepared blood sample.
2. The Prior Art
In conducting tests on samples of biological liquids, such as blood, urine and other body liquids, it is common to provide the sample to an automated analyzer in a sealed test tube or vial. Upon receiving the test tube, the analyzer automatically transports it to an aspiration station where the sharpened tip of an aspiration probe pierces the seal (typically a rubber stopper) and enters the sample volume. After aspirating a portion of the sample from its container, relatively tiny aliquots of the sample, each having a volume of between, say, 1 and 30 microliters, are subsequently dispensed to different reaction chambers and/or baths within the analyzer for processing and/or analysis. While some quantitative analyses, e.g., red and white blood cell counts, require extreme precision in the accuracy of the sample volume dispensed, other, more qualitative, analyses do not require such precision in the dispensing volume.
In general, the liquid aspirating and dispensing apparatus of automated blood analyzers is one of two types: (i) those that aspirate a blood sample into a blood sampling valve or “BSV” that serves to segment the aspirated liquid into multiple precise aliquots for subsequent dispensing, and (ii) those that use a precision syringe pump connected to the aspiration probe to both suction out a portion of the sample from its container, and then expel or dispense multiple metered volumes of the aspirated sample though the same aspiration probe to a reaction chamber or bath. The latter type of aspirating/dispensing apparatus is often referred to as a “suck and spit” apparatus, for obvious reasons.
In many of the automated hematology instruments manufactured by Beckman Coulter, Inc. (e.g. the Models MAXM™ and LH750™ blood analyzers), multiple BSVs are used to segment precise volumes of an aspirated blood sample for analysis. A typical BSV used in these instruments takes the form of a multi-element shear valve assembly, comprising two or more confronting planar pads or plates that are selectively movable relative to each other to define two different configurations; viz., (a) a “loading” configuration in which an aspirated blood sample can be transmitted entirely through the internal and external passageways of the valve assembly, and (b) a “segmenting/dispensing” configuration in which precise volumes of sample passing through certain passageways of the valve assembly (referred to as the “aliquoting chambers”) are segmented or isolated from the rest of the blood sample filling the valve assembly, and positioned to be chased or expelled from the valve by another liquid, such as a diluent or reagent that is used in the sample preparation process. Often, one of the pads has a bore hole passing through it that defines a relatively small aliquoting chamber; alternatively, a planar surface of one of the valve pads is provided with a surface groove that precisely defines, together with a confronting planar surface of an adjacent pad, a desired volume of sample liquid to be dispensed. It is also common that a BSV is provided with one or more external loops of tubing that selectively communicate with internal passages of the BSV, the internal volume of these loops define additional, relatively large, aliquoting chambers that become filled with blood when the valve is in its loading configuration. Various types of BSVs of the type described have been devised, and many have been disclosed in the patent literature; see, e.g., the disclosures of the commonly assigned U.S. Pat. Nos. 4,896,546; 5,158,751; and 5,460,055.
In terms of the preciseness of the sample volumes dispensed, BSVs are the “gold standard.” It is a relatively simple matter to size the aliquoting chambers in such devices to achieve a precise volume. But, since BSVs comprise an assembly of precision parts that are both difficult to manufacture and to assemble in a manner such that the valve operates as intended and without leakage, BSVs add considerable cost to a liquid dispensing system. Further, due to the physical size of conventional BSVs and their related hardware, these devices are commonly located some distance from the sample-aspiration probe of the analyzer, and an appropriate length of tubing is used to connect the probe with each sampling valve. Thus, it will be appreciated that a relatively sizable volume of sample (e.g., about 250-300 microliters) must be aspirated from the sample container in order to fill a series of BSVs and their interconnecting tubing. Typically, out of the aspirated sample volume, no more than about 30% is ever used for the analysis, with the remainder being eventually flushed to waste. In addition to being located at a considerable distance from the aspiration probe, BSVs are often located at a considerable distance from the reaction chambers and baths that make use of the segmented samples they provide; thus, in addition to requiring a relatively large sample, conventional BSVs require a large volume of a non-reactive diluent or other fluid used to chase the segmented sample volumes through tubing connecting the BSVs and the reaction chambers and baths.
Because of their requirement of relatively large sample volumes, hematology instruments that incorporate BSVs of the above type sometime include an auxiliary aspiration probe that is directly coupled to a BSV without any intervening (sample-consuming) tubing. This auxiliary aspiration probe is non-movable within the instrument, and it is usually located outside the instrument housing so that an open container of sample can be manually presented to the probe for aspiration. Thus, upon manually moving a sample container so that the tip of the auxiliary probe is immersed in the sample, the latter is aspirated directly into the BSV from the probe with little waste of the sample. While this auxiliary probe and BSV assembly can be used to aspirate and precisely segment very small volumes of sample, as may be obtained from infants and newborns, its requirement of a manual presentation of the sample dramatically reduces the throughput of the instrument. Further, the need to present a sample for aspiration in a non-sealed container increases the potential of operator exposure to infectious diseases.
As regards the above-mentioned syringe pump (suck-and-spit) approach to aspirating and dispensing liquid, this approach is clearly less costly and is less complicated than the BSV approach. Further, it is advantageous from the standpoint that it less wasteful of the sample to be analyzed. The syringe pump approach relies on the precision movement of a plunger or diaphragm in a fluid path. As the plunger or diaphragm moves in a pulling (sucking) direction, a negative pressure is produced in the probe, causing the sample to be drawn into the probe and its associated tubing through the probe tip. As the plunger or diaphragm moves in the pushing (spitting) direction, a positive pressure is generated that will dispense (or spit) a portion of the just-aspirated sample through the probe tip. Obviously, the accuracy of this approach to aspirating and dispensing a sample depends on the accuracy of moving the plunger or diaphragm in opposite directions. While stepper-motor controlled syringe pumps are known that can satisfy the volume accuracy requirements of some of the sample-preparation processes to be performed, the task of accurately dispensing samples of ten microliters or less can be problematic, especially when the volume to be dispensed approximates the residual volume in the probe after dispensing. Thus, while the syringe pump approach afford several advantages over the BSV approach, the volume accuracy of each aliquot dispensed is difficult to repeatedly control.
In the commonly assigned U.S. Pat. No. 6,322,752 to I. T. Siddiqui, a “hybrid” apparatus is disclosed for aspirating and dispensing blood samples and the like. In one embodiment, a shear valve assembly (i.e., a BSV) is connected directly to an aspiration probe and is movable therewith. The probe is movable within the instrument in at least two perpendicular planes, one to enable it to move into and out of a sample container to access the liquid therein, and one to enable the probe to move relative to various reaction chambers and baths where it is to dispense precise volumes of the aspirated liquid. In a loading configuration, the shear valve enables the aspirated sample to be drawn through the probe, then through an aliquoting chamber in one of the shearing pads of the assembly, and finally through tubing on the upstream side of the valve assembly to a blood detector located between an aspirating pump and the shear valve. Upon sensing that the leading edge of the aspirated volume has reached a point upstream of the shear valve, the latter is operated in its “segmenting/dispensing” configuration in which it serves to (a) isolate that portion of the aspirate liquid within its aliquoting chamber and (b) trap aspirated liquid within the interior volume of the aspirating probe; in this manner, the assembly provides two precise aliquots of liquid for dispensing. Upon operating the shear valve in its segmenting/dispensing configuration, the trapped aliquots of liquid can be chased from the aliquoting chambers within the shear valve and probe interior by connecting them to different sources of positive pressure (e.g., diluent pumps). At the same time, all of the non-used blood that has been aspirated from the sample container and used to fill the shear valve and the lines upstream thereof is flushed to waste.
While the hybrid apparatus described in the above patent is capable of dispensing highly precise volumes of sample for analysis, this apparatus may be considered disadvantageous in that it can only dispense a relatively small overall volume of blood sample, i.e., that contained in the probe and in the aliquoting chamber of the shear valve. Further, it is wasteful of a considerable amount of sample on the upstream side of the shear valve. Still further, once the shear valve has operated to segment the different aliquots of sample, the aspiration probe can do no more than dispense the volume of sample liquid trapped in the shear valve and in the probe interior; it cannot be used simultaneously to achieve other objectives in the instrument.